Field of the Invention
The present invention relates generally to steam turbines and, more specifically, to an improved control stage nozzle vane for use in partial arc operation.
Description of the Related Art
Steam turbine rotary and stationary blades are arranged in a plurality of rows or stages. Usually, the blades of a given row are identical to each other.
The airfoil or vane portion of each rotary or stationary blade includes a leading edge, a trailing edge, a concave surface and a convex surface. The airfoil shape common to a particular row of blades differs from the airfoil shape of every other row within a particular turbine. Likewise, no two turbines of different designs share airfoils of the same shape. The structural differences in airfoil shape result in significant variations in aerodynamic characteristics, stress patterns, operating temperature, and natural frequency of the airfoil. These variations, in turn, determine the operating life of the blades within the boundary conditions (turbine inlet temperature, compressor pressure ratio, and engine speed), which are generally determined prior to airfoil or vane shape development.
Two adjacent control stage nozzle vanes are illustrated in FIG. 1 and are generally referred to by the numerals 10 and 12. Each has a convex, suction surface 14, 16, respectively and opposite side concave or pressure side surfaces 18 and 20, respectively. Each has a leading edge 22, 24, and a trailing edge 26 and 28, respectively. The straight line distance between the trailing edge 26 and the convex surface 16 is referred to as the "throat" opening and is designated by the reference numeral 30. The "pitch" is the distance between trailing edges of adjacent blades and is designated by the reference numeral 32. The gauging of a blade is the ratio of throat to pitch, and is a critical parameter in blade design.
At off-peak or low demand times, such as at night, it is not necessary to run the steam turbine at full power, although the prescribed running speed must be maintained. In order to accomplish this, steam turbines are commonly designed to have a plurality of arcs of admission. For example, as schematically illustrated in FIG. 2, a steam chamber is divided into four segments or four arcs of admission 36, 38, 40 and 42. Each arc of admission is provided with a governor valve 44, 46, 48, and 50, respectively, which are, during full operation, opened to allow steam to enter each of the nozzle chambers (arcs of admission may comprise more than one nozzle chamber).
At low demand, it may only be necessary to allow steam to enter through one or a small group of the nozzle chambers. For example, valve 44 can be placed in an open position, while valves 46, 48 and 50 are shut so that all of the steam entering the turbine is entering through the nozzle chamber 36. At this point, it is said that the turbine is operating in "partial arc operation", and in this case the nozzle chamber 36 represents the primary arc of admission.
The first row 52 of stationary blades is referred to as the control stage nozzle vanes. The trailing edges of the nozzle vanes, particularly in the primary arc of admission, have experienced chipping and erosion because they are exposed to a large pressure load during partial load operation. In other words, due to the fact that only one steam chamber is providing a steam inlet, there is a high pressure difference experienced by the control stage nozzle vanes. The damage that results from this pressure load results in higher maintenance cost due to the requirement of more frequent blade replacement or repair.